Graphene - The Substrate for Plastic Electronics

Transparent and conducting electrodes are needed for applications in solar cells and energy conversion platform like water splitting. To date, there are not many types of transparent and conducting electrodes that can be mass produced cheaply. Available on the market are electrode materials like Indium Tin Oxide (ITO) and Fluorine doped tin oxide. The diminishing supply of indium and its rising cost motivates scientist to search for an alternative electrode material. Moreover, ITO is fragile and can neither be used in flexible electronics nor thermally processed at high temperature.

A soft membrane type material that is mechanically tough and flexible is needed. Graphene is a single layer of carbon sheet with the carbon atoms interconnected in a crystalline honeycomb network. It turns out that highly conducting, ultrathin graphene films can be a good substitute for ITO in all carbon-based flexible electronics because of its transparency and flexible nature.

Graphene has been extensively studied due to its unique electronic and mechanical properties as well as its eagerly projected role in the all-carbon post CMOS technological revolution.1-3 Its two-dimensional (2D) aromatic sheet structure as well as its high conductivity, transparency, mechanical strength and flexibility, imparts great advantages on graphene as a candidate material for the development of "plastic electronics."

In principle cost should not be a major issue for the production of graphene since it can be produced by chemical vapor deposition (CVD) using methane as the gas feed, diluted in hydrogen or argon.4,5 Large area growth and roll to roll processing of graphene is now entering into the first stage of commercial production. CVD-deposited graphene films can be transferred onto glass to generate a new generation of transparent and conducting electrodes. Due to its flexible and sensitive characteristics, graphene membrane can be used in touch screen panels of handphones.

Recently, Professor Kian Ping Loh and his colleagues at the Department of Chemistry, National University of Singapore fabricated large-area, continuous, highly transparent and conducting multilayer graphene films with sheet resistance of 200 ¦¸/square by chemical vapor deposition (CVD) method.6 The CVD grown graphene film can be readily transferred onto glass using polydimethylsiloxane (PDMS) stamp approach and was used as the anode for application in organic photovoltaic cells.

Figure 1. CVD deposited graphene can be used as transparent anode in organic solar cell, offering the advantage of flexibility, transparency and high electrical conductivity.

After non-covalent modification with an organic molecule known as pyrene buanoic acid succidymidyl ester (PBASE), the power conversion efficiency (PCE) of the organic solar cells increased from 0.21% of the unmodified films to 1.71 %. This performance corresponds to ~55.2 % of the PCE of an identical device made with indium tin oxide (ITO) anode, e.g., ITO/PEDOT-PSS/P3HT/PCBM/Al (PCE=3.1%). This finding paves the way for the substitution of the ITO anode with low cost graphene film in photovoltaic and electroluminescent devices.

Besides chemical vapor deposition of graphene, graphene derivatives can also be solution-processed.7,8 Chemists usually used the oxidized form of graphene, known as graphene oxide,7 or generate graphene from graphite using intercalation/exfoliation methods. These graphene derivatives show wide ranging solubility in a range of solvents depending on their preparation methods. Solution processing allows graphene to be spin coated or ink-jet printed on any substrates, this is very useful for developing flexible all-carbon electronics circuit on flexible substrates.

Professor Kian Ping Loh and his colleagues have developed high mobility, printable carbon circuit using solution-processed graphene recently.9 Such type of all-carbon based electronics can be thermally processed at temperature as high as 1000¡ãC in vacuum or non-oxidizing environments. The solution-processability of graphene derivatives allows the fabrication of inorganic-graphene or organic-graphene composites10 to be achieved readily using wet chemistry methods.

Graphene hybrid materials, e.g. graphene coated by quantum dots or infra-red dyes, should demonstrate enhanced performance in photovoltaics. The enhancement of photocurrent generation arises from the efficient dissociation of exciton at the graphene-organic dye or graphene-inorganic semiconductor interface, as well as the increase in spectral absorption bandwidth due to the extended conjugation present in graphene.

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